Consumer electronics, such as mobile phones and personal digital assistants (PDAs), continue to decrease in size and price and increase in functionality. As a result, these electronics devices place severe limitations on both the size and cost of the components (such as integrated circuits (ICs) and Micro-Electro-Mechanical Systems (MEMS) devices) contained therein. Radio frequency (RF) filters including bulk acoustic wave (BAW) filters are ubiquitous components in all RF front ends and play a key role in the design of cellular handsets. There has been a continuing effort to provide inexpensive, compact filter and/or duplexer units that must be tuned to precise frequencies. A BAW resonator comprises at least one piezoelectric thin film and at least one pair of a top electrode and a bottom electrode sandwiching the piezoelectric film therebetween. The resonance frequency of a BAW resonator is the frequency for which the mechanical waves propagate in the device. The half wavelength of the propagating waves is equal to the total thickness of the device for a given phase velocity of the mechanical wave in the material. Besides the thickness of the piezoelectric layer, the thickness and material properties of other layers in the BAW resonator also affect the resonance frequency. One process that makes the production of BAW filters challenging is to guarantee an adequate accuracy of deposited film thickness values across the whole wafer from run to run, in order to keep the frequency tolerance range as low as about 0.1%. Final frequency trimming of passivation layer at the top surface of resonator based on measured frequency distribution is usually necessary to achieve a decent yield.
A typical example of BAW filter is the so-called ladder filter, which often includes a combination of several resonator pairs, one placed in series with the signal to be filtered and the other shunting the signal to be filtered. The two resonators have slightly different resonance frequencies, usually a few percent apart and are usually made the same except for adding a thin mass loading layer to one or the other to shift slightly its resonant frequency compared to the other of the pair. BAW resonators are also used as components of filters making up duplexers in a mobile phone, which in turn includes a transmitter (Tx) filter and a receiver (Rx) filter. A method of fabricating both Tx and Rx filter side by side on a single substrate is disclosed in U.S. Pat. No. 6,407,649. The required frequency difference between Tx and Rx filters is generated by providing a tuning layer in the stack of either of filters. However, in many cases, for example, for wideband code division multiple access (WCDMA) Band I application, the center frequency of Tx band is approximately 10% lower than that of Rx band. In order to achieve optimal filtering performance, the layered stack composition of Tx and Rx filter is quite distinct, and separate frequency tuning of Tx and Rx filters is required, but not practically viable for two dies with distance smaller than 5 mm. In accordance with the demands for smaller size and multi-functionality in mobile electronic products, much effort is being invested in developing the multi-band filter module, comprising, for example, a Global Positioning System (GPS) filter with a passband centered at 1.575 GHz, and a code division multiple access (CDMA) filter with a passband centered at 1.960 GHz. In U.S. Pat. No. 6,518,860, piezoelectric films with different thicknesses have to be deposited separately, thus significantly increasing processing complexity and manufacturing cost. Similar to the problem mentioned earlier, frequency trimming of side by side filters at different bands is not feasible. Overall, the fabrication of BAW duplexer and/or multi-band BAW filters on a single substrate requires compromises in filter performance and manufacturing yield as well as expensive process development.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.